As device dimensions in semiconductor integrated circuits (ICs) continue to shrink, low dielectric constant (low-k) materials are needed as interlevel dielectrics (ILD) to mitigate issues caused by reduced line width and line-to-line distances such as increasing RC-delay. To satisfy the technical requirements imposed by, for example, the microelectronics roadmap (where ultra-low k values <2 are specified), future generation ILDs will likely incorporate porous materials for use as low-k materials. However, the pores of these materials, typically on the order of angstroms to a few nanometers and connected to each other at elevated porosities, can trap moisture, gas precursors, and other contaminants in subsequent processes, making practical pore-sealing techniques essential to ultra low-k implementation.
To be useful for semiconductor integrated circuit applications, a pore-sealing coating should be conformal to the 3D topology of patterned ILD films. In addition, at the 65 nm or smaller technology node, it should be less than several nm thick so that its impact on the overall ILD k value is negligible. These requirements exclude many thin film techniques including, for example, PVD and CVD. One exception is atomic layer deposition (ALD), for which the coatings are inherently conformal and precisely controlled at sub-nm thicknesses.
Generally, ALD processes form a monolayer of precursor molecules chemically adsorbed on a surface to be coated. Then, other molecules, for example, in gaseous form, are introduced to react with that monolayer so that one atomic layer of the material desired is deposited. Normally there are several layers of molecules adsorbed on the surface. The first layer is a chemically adsorbed layer and has a strong bond with the surface. The next layers are physically adsorbed layers and are weakly bonded with each other. ALD makes use of this difference between chemical adsorption and physical adsorption. At elevated temperatures or reduced partial pressures, over broad ranges, the weakly bonded physically adsorbed molecules are removed leaving only the saturated chemisorbed monolayer on the surface. For example, the chamber can be purged by inert gas or evacuated to a low pressure, to form a saturated conformal monolayer on the sample surface. Then, the second gas is introduced to react with the precursor molecules and form an atomic layer of thin film.
Problems arise using conventional ALD on a porous substrate because conventional methods allow molecules to penetrate into the internal porosity of the ILD material, filling pores and drastically increasing the effective k value. Because ALD is a surface adsorption-based deposition process, thin film formation can take place wherever gas precursor adsorption occurs, including throughout the network of connected internal porosity. FIG. 1 depicts a cross section of a portion of a porous material 110 that includes a plurality of pores 120. Conventional ALD forms a barrier layer 130 on the surface of porous material 110, but also forms film 135 within the internal pores thereby increasing the effective k value of porous material 110. At small ILD feature dimensions, even short precursor exposure times that reduce the ALD penetration depth to, for example, 10 nm, fills a large percentage of the ILD pores.
Thus, there is a need to overcome these and other problems of the prior art to provide barrier layers and methods of forming barrier layers that are conformal and localized to the surface of a porous material.